1. Field of the Invention
This invention relates to computers employing power management systems and particularly to circuits and methods for allowing a computer to enter a power conserving mode while executing a host signal processing modem.
2. Description of Related Art
Host signal processing (HSP) modems reduce the cost of providing modem functions to a computer system by utilizing the processing power of the central processing unit (CPU) of a host computer rather than including a dedicated digital signal processor (DSP) in modem hardware. For example, typical LISP modem hardware includes: an interface for connection to telephone lines; a digital-to-analog converter (DAC); an analog-to-digital converter (ADC); a buffer for temporary storage of samples representing signals transmitted and received on the telephone lines; and an interface for connection to the host computer. During operation with an active connection over the telephone lines, the HSP modem hardware periodically interrupts the host CPU which in response executes HSP modem software. The HSP modem software retrieves received samples from the HSP modem hardware and converts the received samples to data for a communications application. The HSP modem software also retrieves data from the communications application, converts the data into samples representing the signal to be transmitted on the telephone lines, and writes those samples to the HSP modem hardware.
One disadvantage of an HSP modem can arise from the interaction of the HSP modem with a power management system such as often employed to conserve battery power in a portable computer. One type of power management system tracks a computer""s activity and when the computer is inactive for a predetermined period, places the computer in a power-saving (sleep) mode in which the system clock for the central processing unit (CPU) is slowed or stopped. The power management system turns on or increases the frequency of the system clock to place the computer in a normal operating mode when the computer resumes activity. An HSP modem can effectively disable a power management system by maintaining periodic interrupts to the main CPU. The power management system senses the periodic interrupts as system activity and keeps the computer in a normal operating mode. This is appropriate during an active telephone connection because the system is actively processing information and probably should not be in the power-saving mode. But, when there is no active modem connection, the HSP modem and the computer system may not be performing any useful activity, and the computer should be place in power-saving mode to conserve power. The HSP modem can be turned off to stop the periodic interrupts and allow the system to enter sleep mode, but this comes at the cost of losing modem functions. For example, while there is no active connection, the HSP modem may be waiting for a remote device to call in and establish a connection. Turning off the HSP modem disables the ability to handle an incoming call. Accordingly, an HSP modem system is sought that allows a power management system to place a computer in a power-saving mode when there is no active connection and is still able to respond to incoming calls received while in the power-saving mode.
In accordance with the invention, an HSP modem or other HSP communication system includes a device that connects to telephone or other communication lines and is operable in a normal mode and a wait mode. In the normal mode, the device generates periodic interrupts of the host processor; and in response to the periodic interrupts, a host CPU executes software for processing information transferred via the device. In the wait mode, periodic interrupts to the host CPU are suspended but a communication interface such as a DAA (digital/analog adapter) circuit is active and interrupts the host computer for an incoming signal on the communication lines. Accordingly, while in wait mode, the device does not generate periodic interrupt activity, and a power management system can place the host CPU in a power-saving mode while waiting for an incoming communication signal. In response to the incoming signal, the device is switched to the normal mode in which the device generates periodic interrupts to the host CPU for processing information transferred via a newly created modem connection. The interrupts cause the power management system to switch the host CPU back to normal operating mode if the host CPU was operating in power-saving mode. Accordingly, a computer system can be in a power-saving mode while waiting for an incoming ring signal and still respond to the ring signal to establish a modem connection.
One HSP modem system in accordance with the invention is implemented using a multiplexer and an HSP modem integrated circuit (IC). An interrupt signal from the HSP modem IC and a ring signal from DAA circuit provide input signals to the multiplexer. The output terminal of the multiplexer is coupled to an interrupt line of the host processor. To place the HSP modem hardware in the wait mode, the HSP modem IC is shut off or idled and the multiplexer selects the ring signal as an interrupt to the host processor. Shutting off the HSP modem IC halts periodic interrupts to the host processor and allows a power-conservation system to switch the host processor to power-saving mode. In the wait mode, a ring signal interrupts the host processor causing the host processor to execute HSP modem software. The HSP modem software turns on the HSP modem IC and causes the multiplexer to provide the interrupts from the HSP modem IC to the host processor.
HSP modems or communication systems according to embodiments of the invention may also include a time-out counter either in software or in hardware that places the HSP modem hardware in wait mode after a predetermined period with no incoming signal from a communication line and no service request from a communication application. The HSP modem hardware reactivates in response to a ring signal for an incoming call or the host computer writing to the configuration register when the host computer initiates a modem connection.